To achieve high efficiencies and high outputs, nitride semiconductor light emitting devices (hereinafter also referred to as LEDs (Light Emitting Diodes)) designed for white lighting devices are being improved in crystalline structures and device structures, and higher internal quantum efficiencies and higher light extraction efficiencies are being realized.
Where InGaN-based crystals are grown, a sapphire substrate is often used, because it is inexpensive and stable. A crystal growth with high crystallinity can be performed on a sapphire substrate with a low-temperature buffer. However, being an insulator, a sapphire substrate does not have conductive properties and is low in thermal conductivity. Therefore, electrodes cannot be formed on the back face side of a sapphire substrate, and p- and n-electrodes need to be formed on the nitride semiconductor side. Therefore, the tendency to cause higher series resistance and the low heat release properties during a high-power operation become problems in achieving even higher efficiencies and outputs.
A thin-film InGaN-based LED is known as one of the LED structures that eliminate the above problems and improve luminous efficiencies and outputs. Such a thin-film InGaN-based LED transfers LED structural crystals grown on a sapphire substrate onto another supporting substrate such as a Si substrate, a copper substrate, or a gold substrate. As devices are formed after the transfer onto a supporting substrate having conductive properties and high thermal conductivity, the current spread becomes larger by vertical energization, and the electric conductive properties are improved. Further, the heat release properties are also improved.
Also, by forming a structure that has an n-layer as an upper face through a transfer and extracts light from the n-layer side, a transparent electrode for diffusing current becomes unnecessary for the n-layer having lower resistance than a p-layer. Since light is not absorbed by a transparent electrode, the light extraction efficiency becomes higher. This process of transfer includes a process to bond crystals (epitaxial crystals) formed through an epitaxial growth to the supporting substrate, and a lift-off process to detach the epitaxial crystals from the sapphire substrate. The bonding process may involve a plating technique or a joining technique utilizing weight and heat, and the lift-off process may involve a laser lift-off technique utilizing thermolysis of an interface caused by a laser or a chemical lift-off technique.
In such a thin-film LED structure, the difference in refractive index between the surface of a GaN substrate and the external air is as large as 2.5 times where only a laser lift-off process has been carried out, and the light reflection from the boundary face lowers the light extraction efficiency.
To counter this problem, a technique of producing concavities and convexities on the surface of a chip has been suggested. The concavities and convexities are formed by regrowing, polishing, and etching an n-type nitride semiconductor layer. According to a method for simple formation, concavities and convexities are formed by roughening the surface through alkaline etching performed on the n-layer on the upper face of a GaN substrate on a supporting substrate. In this manner, the light extraction efficiency is made higher.
By the conventional alkaline etching, however, the sizes of the concavities and convexities cannot be made larger in many cases, even though the entire film thickness is reduced by prolonging the etching period. Therefore, there is a demand for an etching control method by which such concavities and convexities as to improve the light extraction efficiency can be formed on the surface of a GaN layer.